Idaho semi-weekly world. [volume] (Idaho City, Idaho Territory) 1875-1908, 1887-10-25
ToRNADoEs AND CYCLONES


When the conditions of atmospheric
instability have given birth to tor
nado. the fact is announced to the OBJ
server by a sudden gathering of dark.
swift whirling clouds, from which de-
pend writhing. serpent-iike body
formed of condensed vapor. This
writhing column extends rapidly
downward until it touches the earth
When 1t attains the surface 1t becomes
audible from the violent rending nc
lions which it creates upon that sur
face. As soon as the whirl is created
1t begins TO move away generally
toward the northeastern the evident
reason that the upper cold layer of air
against which IL originates has. in the
northern hemisphere, q movement in
that direction. In its path over the
surface the circling movement OF the
writhing air and the sucking action OF
the partial vacuum In tho central por-
ton OF the shaft combine to bring
about an extreme devastation. On
the outside Of the whirl the air, which
rushes in circling path toward the
vortex, overiurns all movable objects,
and in the center these objects. If not
TOO heavy. are sucked up as by great
air-pump. Thus the roofs OF
houses bodies Of men and animals
may be lifted to great elevations, until
they are tossed By the tumultuous move
ments beyond the limits OF the ascend
Ing currents and fall upon the earth.
Where the center Of The whirlwiud
passes over building the sudden de-
crease in the pressure OF the outer air
often causes the atmosphere which is
contained within the walls suddenly
TO press against the sides OF the struci-
urc so that these sides are quickly
driven outward as II by charge Of
gunpowder. IL IL not unlikely that
the dimuniiion Of pressure brought
about by the passage II the interior Of
tho whirl over building may be about
Ms mush as Is indicated by the fall Of
four inches in the barometer This is
equivalent 10 change In the pressure
amounting 10 about three hundred
pounds TO tho square foot This force
operates to burst out the walls Of n
building. IL is not improbable that
the dinlinniion Of pressure may be
much greater than this, but even the
amount named Is sufficient to account
structures which these devastating
movements encounter in the western
parts Of the United States.
The way ia which these tornado
whiris are formed differs in certain es-
soniial particulars from the way In
which whrlwinds are created, ms has
been we'll shown by Prof Ferrel The
most important points of difference are
rs follows; The dusl-whrls are due to
the heating Of thin layer Of air next
the ground. The small mass If this
layer prevents its upward whirliug
from bringing about any powerful
movements of the atmosphere. In the
tornado the heat of the lower mir has
different origin. When cyclone
rasses over the surface of the country.
certain peculiar movements Of the ut
mosphere which II produces bring
large volumes OF the warm aud moist
ened air TO the eNrth's surface and
overlay them by cool stratum. This
layer Of warm. moist air tends TO rise
mp for the same reason that the thin
layer OF dry air which forms tho dust
whirl is in,peled upward. but on ac-
count of It's great mass the intensity OF
tho upward urgence is far greater In
the sand-whrl The upward motion be-
gins close 10 the earth's surface, fol
the reason that the stratum which is
impeled upward is very thin. but in
the tornado the stratum of heated
air Is usually about thousand
feet thick; therefore its whirling
action naturally originates at the up
per surface OF the hot layer. for it is at
that point the upward motion begins.
Starting in this upper region, the whirl
extends progressively downward, just
IT'S In the bath-tub the whirl extends
progressively upward from the point
at which the motion originated, until
the wind may touch the surface of the
earth. When these whirls begin they
only involve II small part Of the air
about the point of origin. aud so the
acquired velocity Of the particles whom
they come 10 the oonter is not great,
but gradually they suck air from far
Their and farther away. A. the Held of
supply becomes larger, aud the par
tieles move from a greater distance.
they approach hat center with greater
speed. and the spiral widens and turns
with accelerated velocity.
Fortunately the paths of tornadoe.
are ordinarily very narrow--the wild.
est have diameter of less than tive
miles; the narrowest of only forty feet.
In most cases a tornado is seriously
destructive over width not exceed.
ing live hundred feet. The length
Il the tornado's path across
the country does not commonly exceed
thirty miles. and 1t generally traverse.
the distance in about an hour. Whel
The upward corkscrew motion of Tia,
outer par, Of the spiral aud the swifte
uprush of the air through the centra
shaft have drained away the most o
the warm air which gave birth to tn,
motion. the tornado dies away. Tn,
equilibrium of the air-mssscs is for
time restored. the heavier air has fal
len down upon the surface. and tn,
WARM air, spreading literally as 1t at
iains the level to which il tends, comes
into state Of quiet Assuming thu
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